The present invention relates to the field of plasma spray guns and particularly to a plasma spray gun designed to be very rugged and suitable for extended high power operation.
In typical plasma flame spraying systems, an electrical arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electrical arc and is excited thereby to temperatures of up to 30,000.degree. F. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-actylene flame. A typical plasma flame spray gun is described in U.S. Pat. No. 3,145,287.
The electrical arc of such plasma spray guns, being as intense as it is, causes nozzle deterioration and ultimate failure. One cause of such deterioration is the fact that the arc itself strikes the nozzle at a point thereby causing instantaneous local melting and vaporizing of the nozzle surface. Deterioration is also caused by overheating the nozzle to the melting point so that part of the nozzle material flows to another location, which may eventually cause the nozzle to become plugged. There are varying degrees and rates associated for each cause for nozzle deterioration. Experience has shown that wall erosion, ultimately causing the coolant to burst through the nozzle wall, is another cause for nozzle failure. When the wall bursts, coolant water is released into the arc region, resulting in an intense electric arc, causing parts to melt. Once a meltdown has occured, gun repair can be very costly. The nozzle deterioration and failure problem is particularly severe at high power levels.
In seeking to overcome this problem, plasma flame spray guns have been designed with easily changed water cooled nozzles. During operation, water coolant is pumped through passages in the nozzle to cool the nozzle walls. Even so, gradual, or sometimes rapid deterioration occurs and, as a precaution against failure, the nozzles are usually replaced after a given number of hours of service. This practice of replacing the nozzle periodically, however, is quite costly because the interchangeable nozzles are fairly expensive and many nozzles with considerable life remaining are thereby discarded.
Another cause of failure is believed to be the fact that the gun parts are placed under more stress in extended service applications causing them to warp resulting in uneven wear, possible water leakage and more rapid failure. A similar problem is distortion of the gun during re-assembly, resulting from inadvertent over- or under-tightening of the bolts that hold the gun parts together.
One particularly troublesome mode of failure in all plasma spray guns is caused by coolant leakage. This typically occurs when a seal between a coolant passage and the plasma passage fails. When this occurs, the cooling fluid enters the region where the arc is produced, causing an electrical short circuit which usually results in a meltdown of gun parts. Even a minor leak upsets the arc operation resulting in rapid deterioration of the cathode and anode. A costly repair is thereafter required to again place the gun into service.
In view of the above-mentioned problems associated with prior art plasma spray guns when placed into heavy duty operation, it is the primary object of the present invention to provide a plasma spray gun capable of extended operation.
It is another object of the present invention to provide a heavy duty plasma spray gun capable of extended operation which requires relatively little routine maintenance to prevent failures.
It is yet another object of the invention to provide a heavy duty plasma spray gun with a readily perceptible indication to operators that an internal leak in the cooling system has occurred and that there is a danger of a meltdown due to that leak.
It is still a further object of the invention to provide a heavy duty plasma spray gun with rugged construction to prevent heat distortion of the gun parts during extended operation.
It is yet another object of the invention to provide a mechanism to assure that possible debris and cooling fluid do not enter the inert gas delivery system of the gun thereby preventing damage which might be caused thereby.